Measurement of water quality, Biology tutorial


Monitoring of environmental quality parameters is the key activity in managing environment, restoring polluted environments and expecting effects of man-made changes on environment. It comprises collection of water samples and their analyses to find out quality of water. Qualities generally analyzed for in water samples comprise Chemical constituents: color and turbidity, organic constituents, metals (zinc, iron, heavy metals), phosphorus, nitrogen, solids, arsenic, microbial load (bacteria, viral and parasite inputs) and physical characteristics such as pH, color, turbidity, dissolved oxygen, etc

Collection of Water Samples:

The first step in understanding chemistry of freshwaters is to take samples and analyze them for chemical constituents that are of interest. Freshwaters are astonishingly difficult to sample as they are hardly ever homogeneous and their quality differs during day and during year. Additionally the most representative sampling locations are frequently at the distance from shore or bank increasing logistic complexity.

Sampling of Rivers:

Filling the clean bottle with river water is a extremely easy task, but the single sample is only representative of that point along river the sample was taken from and at that point in time. Understanding chemistry of the whole river, or even a important tributary, needs prior investigative work to understand how homogeneous or mixed flow is and to find out if quality changes during course of a day and during the course of year. Approximately all natural rivers will have very important patterns of change through the day and through the seasons.

Many rivers also have a very large flow that is unseen. This flows through underlying gravel and sand layers and is called the hyporheic zone. How much mixing there is between the hyporheic zone and the water in the open channel will depend on a variety of factors, some of which relate to flows leaving aquifers which may have been storing water for many years.

Ground-waters Samples: Ground waters by their very nature are frequently very hard to access to take the sample. As the consequence the most of the groundwater data comes from samples taken from springs, wells, water supply boreholes and in natural caves.

Lakes Samples: Lakes and ponds can be extremely large and support the complex ecosystem in which environmental parameters differ widely in all three physical dimensions and with time. Large lakes in temperate zone frequently stratify in warmer months into the warmer upper layers rich in oxygen and the colder lower layer with low oxygen levels.

When stratification takes place it not only influences oxygen levels but also several associated parameters like iron, phosphate and manganese that are all changed in their chemical form by change in redox potential of environment.

Determination of water quality:

a) pH of Water Samples: This can be found using universal indicator or pH probe. Dip universal indicator test paper in water sample and compare the color produced with the color with chart of the indicator. In the case of pH probe, it first rinsed with distilled water and dipped in the water with the pH read off pH meter.

b) Water Current: This can be estimated by noting the time it takes the floating object to cover known distance.

c) Total Suspended Solids: This can be estimated by filtering the known amount of water through the pre weighed filter paper, drying filter paper at 105oC and weighing dried filter paper. Difference between initial weight of the filter paper and final weight of filter paper is the total suspended solid.

d) Dissolved Oxygen: This is found using Winkler method or by using electronic oxygen meter. Winkler method engages adding 1ml of each of MnSO4 and alkaline iodine azide in 125ml of water sample. This is followed by addition of 1ml conc H2SO4 and tilting of bottle until brown precipitate forms. 50ml of aliquot is then titrated against 0.025N solution of sodium thiosulphate until disappearance of blue color using starch as indicator.

DO (mg/ml) = (ml of titrant X Normality of titrant X equi wt of O2 X 1000) /ml of sample

f) Nitrite Level: This can be done based on Diazotization reaction. It engages mixing 40ml of water with 2ml of sulphanilamide solution. After shaking mixture and permitting it to settle for 10 minutes, 2ml of N-(naphyth) ethylene dimine dihydrochloride is added mixed thoroughly. Resulting purple azo dye is measured at 543nm

Factors which Affect Water Quality:

Water chemistry between systems differs tremendously. The main sources of variation are atmospheric inputs, anthropogenic inputs and toxicity.

Atmospheric inputs: Oxygen is possibly the most significant chemical constituent of surface water chemistry, as all aerobic organisms require it for survival. It enters water mostly using diffusion at the water-air interface. Oxygen's solubility in water reduces as water temperature increases. Fast, turbulent streams expose more of water's surface area to the air and tend to have low temperatures and therefore more oxygen than slow, backwaters. Oxygen is a byproduct of photosynthesis, so systems with high abundance of aquatic algae and plants may also have high concentrations of oxygen during the day.

Most other atmospheric inputs come from man-made or anthropogenic sources the most important of which are the oxides of sulphur generated by burning sulphur rich fuels like coal and oil that give rise to acid rain. Chemistry of sulphur oxides is complex both in atmosphere and in river systems. However the effect on overall chemistry is simple in that it decreases pH of the water making it more acidic. pH change is most marked in rivers with extremely low concentrations of dissolved salts as these can't buffer effects of the acid input.

Anthropogenic Inputs: The majorities of rivers on planet and several lakes have received or are receiving inputs from human-kind's activities. In industrialized world, several rivers have been extremely seriously polluted, at least during 19th and the first half of the 20th centuries.

Toxicity: In most environmental conditions presence or absence of the organism is determined by the complex web of interactions only some of which will be associated to assessable chemical or biological parameters. Most chemical constituents favor some organisms and are less favourable to others. However there are some cases where the chemical constituent exerts the toxic effect, i.e. where concentration can kill or severely inhibit normal working of organism.

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